Light-emitting device and electronic device including the same

ABSTRACT

A light-emitting device is provided, which includes a circuit board, a plurality of light-emitting elements, a first reflective element, and a second reflective element. The light-emitting elements are arranged on the circuit board. The first reflective element is disposed on the circuit board. The second reflective element is disposed on the circuit board. The first reflective element has a first reflectivity R1. The second reflective element has a second reflectivity R2. The first reflectivity R1 is different from the second reflectivity R2. The first reflectivity R1 and the second reflectivity R2 satisfy the following formula: 0&lt;|(R1−R2)|/Max (R1, R2)&lt;20%.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of pending U.S. patent applicationSer. No. 17/344,580, filed Jun. 10, 2021 and entitled “BACKLIGHT MODULEAND ELECTRONIC DEVICE INCLUDING THE SAME”, the entirety of which isincorporated by reference herein.

BACKGROUND Technical Field

The present disclosure relates to an electronic device, and inparticular it relates to a backlight module of an electronic device.

Description of the Related Art

Electronic products with panels, such as displays, smartphones, tabletcomputers, notebook computers, and televisions, have becomeindispensable necessities in modern society. With the flourishingdevelopment of these electronic products, consumers have highexpectations regarding their quality, functionality, or price.

However, these electronic products have not yet met consumerexpectations in various aspects. There are still some issues existing inthe electronic product. For example, in an electronic device with abacklight module, the tape disposed adjacent to the light-emittingelements may affect the extraction efficiency of the backlight module.The development of a structural design of the backlight module that canimprove the extraction efficiency is still one of the goals in thecurrent industry.

SUMMARY

In accordance with some embodiments of the present disclosure, alight-emitting device is provided. The light-emitting device includes acircuit board, a plurality of light-emitting elements, a firstreflective element, and a second reflective element. The plurality oflight-emitting elements are arranged on the circuit board. The firstreflective element is disposed on the circuit board. The secondreflective element is disposed on the circuit board. The firstreflective element has a first reflectivity R1. The second reflectiveelement has a second reflectivity R2. The first reflectivity R1 isdifferent from the second reflectivity R2. The first reflectivity R1 andthe second reflectivity R2 satisfy the following formula:0<|(R1−R2)|/Max (R1, R2)<20%.

In accordance with some embodiments of the present disclosure, anelectronic device is provided. The electronic device includes a displaypanel and a light-emitting device. The light-emitting device is disposedbelow the display panel. The light-emitting device includes a circuitboard, a plurality of light-emitting elements, a first reflectiveelement, and a second reflective element. The plurality oflight-emitting elements are arranged on the circuit board. The firstreflective element is disposed on the circuit board. The secondreflective element is disposed on the circuit board. The firstreflective element has a first reflectivity R1. The second reflectiveelement has a second reflectivity R2. The first reflectivity R1 isdifferent from the second reflectivity R2. The first reflectivity R1 andthe second reflectivity R2 satisfy the following formula:0<|(R1−R2)|/Max (R1, R2)<20%.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 is a partial top-view diagram of a backlight module in accordancewith some embodiments of the present disclosure;

FIG. 2A is a cross-sectional diagram of the backlight module taken alongsection line A-A′ in FIG. 1 in accordance with some embodiments of thepresent disclosure;

FIG. 2B is a cross-sectional diagram of the backlight module taken alongsection line B-B′ in FIG. 1 in accordance with some embodiments of thepresent disclosure;

FIG. 3 is a spectrum of the light source that is used for themeasurement of reflectivity and chromaticity in accordance with someembodiments of the present disclosure;

FIG. 4 is a cross-sectional diagram of a light-emitting element of abacklight module in accordance with some embodiments of the presentdisclosure;

FIG. 5 is a partial top-view diagram of a backlight module in accordancewith some embodiments of the present disclosure;

FIG. 6 is a partial top-view diagram of a backlight module in accordancewith some embodiments of the present disclosure;

FIG. 7 is a partial top-view diagram of a backlight module in accordancewith some embodiments of the present disclosure;

FIG. 8 is a partial top-view diagram of a backlight module in accordancewith some embodiments of the present disclosure;

FIG. 9 is a partial top-view diagram of a backlight module in accordancewith some embodiments of the present disclosure

FIG. 10 is a partial top-view diagram of a backlight module inaccordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The backlight module and electronic device of the present disclosure aredescribed in detail in the following description. It should beunderstood that in the following detailed description, for purposes ofexplanation, numerous specific details and embodiments are set forth inorder to provide a thorough understanding of the present disclosure. Theelements and configurations described in the following detaileddescription are set forth in order to clearly describe the presentdisclosure. The embodiments are used merely for the purpose ofillustration. In addition, the drawings of different embodiments may uselike and/or corresponding numerals to denote like and/or correspondingelements in order to clearly describe the present disclosure. However,the use of like and/or corresponding numerals in the drawings ofdifferent embodiments does not suggest any correlation between differentembodiments.

The present disclosure can be understood by referring to the followingdetailed description in connection with the accompanying drawings. Itshould be noted that, in order to allow the reader to easily understandthe drawings, several drawings in the present disclosure only depict aportion of the electronic device, and the specific elements in thedrawings are not drawn to scale. In addition, the number and size ofeach element in the drawings are only for illustration and the scope ofthe present disclosure is not limited thereto.

Throughout the present disclosure and the appended claims, certain termsare used to refer to specific elements. Those skilled in the art shouldunderstand that electronic device manufacturers may refer to the sameelement with different names. The present disclosure does not intend todistinguish between elements that have the same function but differentnames. In the specification and claims, the terms “comprising”,“including”, “having” and the like are open-ended phrases, so theyshould be interpreted as “including but is not limited to . . . ”.Therefore, when the terms “comprising”, “including” and/or “having” areused in the description of the present disclosure, they specify thecorresponding features, regions, steps, operations and/or components,but do not exclude the existence of one or more corresponding features,regions, steps, operations and/or components.

Directional terms mentioned in the present disclosure, such as “upper”,“lower”, “front”, “rear”, “left”, “right”, etc., are only the directionsreferring to the drawings. Therefore, the directional terms are used forillustration, not for limiting the scope of the present disclosure. Thedrawings depict general features of methods, structures, and/ormaterials used in particular embodiments. However, these drawings shouldnot be interpreted as defining or limiting the scope or propertyencompassed by these embodiments. For example, for clarity, the relativesizes, thicknesses, and positions of the various layers, regions, and/orstructures may be reduced or enlarged.

When a corresponding component (such as a layer or region) is referredto as “(disposed or located) on another component”, it may be directly(disposed or located) on another component, or there may be othercomponents between them. On the other hand, when a component is referredto as “directly (disposed or located) on another component”, there is nocomponent existing between them. In addition, when a component isreferred to as “(disposed or located) on another component”, the twohave an upper-lower relationship in a top-view direction, and thiscomponent may be above or below another component, and the upper-lowerrelationship depends on the orientation of the device.

The terms “about”, “equal to”, “the same as”, “identical to”,“substantially” or “approximately” are generally interpreted as beingwithin 20% of a given value or range, or within 10%, 5%, 3%, 2%, 1% or0.5% of the given value or range.

The ordinal numbers used in the specification and claims, such as theterms “first”, “second”, etc., are used to modify an element, whichitself does not mean and represent that the element (or elements) hasany previous ordinal number, and does not mean the order of a certainelement and another element, or the order in the manufacturing method.The use of these ordinal numbers is used to make a component with acertain name can be clearly distinguished from another component withthe same name. The same words may not be used in the claims and thespecification. Accordingly, the first component in the specification maybe the second component in the claims.

It should be noted that the following embodiments can replace,recombine, and combine features in several different embodiments tocomplete other embodiments without departing from the spirit of thepresent disclosure. The features between the various embodiments can becombined and used arbitrarily as long as they do not violate or conflictthe spirit of the present disclosure.

In the present disclosure, the length and the width of the component canbe measured from an optical microscope image, and the thickness of thecomponent can be measured from a cross-sectional image in an electronmicroscope, but it is not limited thereto. In addition, certain errorsmay exist between any two values or directions used for comparison. Ifthe first value is equal to the second value, it implies that there maybe a 10% error between the first value and the second value; if thefirst direction is perpendicular to the second direction, the anglebetween the first direction and the second direction may be between 80degrees and 100 degrees; if the first direction is parallel to thesecond direction, the angle between the first direction and the seconddirection may be between 0 degrees and 10 degrees.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure belongs. It should be appreciated that,in each case, the term, which is defined in a commonly used dictionary,should be interpreted as having a meaning that conforms to the relativeskills of the present disclosure and the background or the context ofthe present disclosure, and should not be interpreted in an idealized oroverly formal manner unless so defined.

In accordance with some embodiments of the present disclosure, abacklight module and an electronic device including the backlight moduleare provided. The backlight module includes a first reflective elementand a second reflective element that are disposed adjacent tolight-emitting elements and are arranged in a specific configuration. Inaddition, the reflectivity of the first reflective element and thesecond reflective element are designed to follow a certain formula. Withsuch a specific configuration, the extraction efficiency of thebacklight module can be improved. The extraction efficiency is theproportion of light emitted from the backlight module to the displaypanel. Therefore, the performance or reliability of the electronicdevice (e.g., a display device) can be enhanced.

In some embodiments, the electronic device may include a display device,a light-emitting device, a touch device, a sensing device, a tileddevice, or a combination thereof, but it is not limited thereto. Theelectronic device may include a bendable or flexible electronic device.In some embodiments, the electronic device may include light-emittingdiode (LED), liquid crystal, fluorescence, phosphor, quantum dot (QD),another suitable medium, or a combination thereof, but it is not limitedthereto. The light-emitting diode may include, for example, an organiclight-emitting diode (OLED), an inorganic light-emitting diode, such asa mini light-emitting diode (mini LED), a micro light-emitting diode(micro LED) or a quantum dot light-emitting diode (QLED/QDLED), anothersuitable material or any combination of the foregoing, but it is notlimited thereto. In addition, the shape of the electronic device may bea rectangle, a circle, a polygon, an irregular shape, a shape with acurved edge, or another suitable shape. In addition to the displaypanel, the electronic device may also include peripheral systems such asa driving system, a control system, a light source system, a displaydevice will be used as an example to describe the electronic device, butthe present disclosure is not limited thereto.

In some embodiments, an electronic device is provided, and theelectronic device includes a display panel DP and a backlight module 10.The backlight module 10 may be disposed below the display panel. Inother words, the display panel DP may be closer to the viewer than thebacklight module 10. Specifically, refer to FIG. 1 , FIG. 2A and FIG.2B. FIG. 1 is a partial top-view diagram of the backlight module 10 inaccordance with some embodiments of the present disclosure. FIG. 2A is across-sectional diagram of the backlight module 10 taken along sectionline A-A′ in FIG. 1 in accordance with some embodiments of the presentdisclosure. FIG. 2B is a cross-sectional diagram of the backlight module10 taken along section line B-B′ in FIG. 1 in accordance with someembodiments of the present disclosure.

It should be understood that only some elements of the backlight module10 are illustrated in FIG. 1 , FIG. 2A and FIG. 2B for clarity. In someembodiments, additional features or elements may be optionally added tothe backlight module 10. In some embodiments, some features of thebacklight module 10 described below may be optionally replaced oromitted. In addition, some elements illustrated in FIG. 2A and FIG. 2Bare omitted in FIG. 1 for clarity.

As shown in FIG. 1 , FIG. 2A and FIG. 2B, the backlight module 10 mayinclude a circuit board 100, a plurality of light-emitting elements 200,a light guide plate 110, a first reflective element 310 and a secondreflective element 320. It should be understood that, FIG. 1 onlyillustrates one light-emitting element 200 for clarity. The backlightmodule 10 may include a plurality of light-emitting elements 200, andthe light-emitting elements 200 may be arranged on the circuit board 100along a first direction X. In addition, the light guide plate 110 may bedisposed on the circuit board 100. The light guide plate 110 may bepartially overlapped with the circuit board 100 along a normal directionZ of the circuit board 100. Moreover, the first reflective element 310and the second reflective element 320 may be disposed between the lightguide plate 110 and the circuit board 100. In some embodiments, thefirst reflective element 310 may be disposed between the light guideplate 110 and the second reflective element 320.

The circuit board 100 may include a printed circuit board (PCB) orflexible printed circuit (FPC), but it is not limited thereto. Thecircuit board 100 may be electrically connected to the light-emittingelements 200 and control the light-emitting elements 200.

In some embodiments, the light-emitting elements 200 may include, butare not limited to, inorganic light-emitting diodes, micro-LEDs,mini-LEDs. In some embodiments, the light-emitting elements 200 mayinclude a packaging component and a bare die (such as the micro-LED, orthe mini-LED) in the packaging component. In some embodiments, thelight-emitting elements 200 may include surface-mount devices (SMD)packaging of light-emitting diodes, chip-on-board (COB) packaging oflight-emitting diodes, another suitable packaging form, or a combinationthereof, but it is not limited thereto. The detailed structure of thelight-emitting elements 200 will be described in the following context.

Referring to FIG. 2A and FIG. 2B, in some embodiments, the backlightmodule 10 may further include a third reflective element 120 disposedbelow the light guide plate 110. In other words, the light guide plate110 may be between the third reflective element 120 and the viewer. Insome embodiments, the third reflective element 120 may be disposedadjacent to the circuit board 100, and the third reflective element 120may be or not be in contact with the circuit board 100. In someembodiments, the light guide plate 110 can guide a light emitted fromthe light-emitting elements 200 to the display panel DP. The reflectiveelement 120 can be used to reflect part of the light emitted from thelight-emitting element 200 and transmitted out from the light guideplate 110, and/or reflect the light escaped from the light guide plate110 back into the light guide plate 110 again.

It should be understood that although the light guide plate 110 isomitted in FIG. 1 , the light guide plate 110 would be disposed in frontof the light-emitting element 200 in FIG. 1 . In other words, thecircuit board has a side 100S, the side 100S is adjacent to the lightguide plate 110 and is extending along the first direction X, part ofthe light guide plate 110 may overlap with the third reflective element120, and another part of the light guide plate 110 may overlap with theside 100S along the normal direction Z of the circuit board 100. Itshould be noted that, the expression “the side (or the element)extending along a direction A (such as the first direction X)” meansthat there may be 20 degrees difference between an extending directionof the side (or element) and the direction A.

In addition, as shown in FIG. 1 , the circuit board 100 may have aplurality of light-reflecting regions 100R, and the light-emittingelements 200 may emit a light toward the light-reflecting regions 100R.In some embodiments, the light guide plate 110 may be overlapped withthe light-reflecting regions 100R along the normal direction Z of thecircuit board 100. In some embodiments, a virtual line VL1 may bethrough a site of the light-emitting element 200 with a maximum widthW₂₀₀, the light-reflecting region 100R may be defined by a regionenclosed by the virtual line VL1, a virtual line EX1, a virtual line EX2and a side 100S of the circuit board 100. Specifically, in someembodiments, when the light-emitting elements 200 includes the packagingcomponent, the site of the light-emitting element 200 with the maximumwidth W₂₀₀ may refer to a site of the packaging component of thelight-emitting element 200 with the maximum width. In addition, the sitewith the maximum width W₂₀₀ may be substantially extended along thefirst direction X. The site of the light-emitting element 200 with themaximum width W₂₀ has two ends, the virtual line EX1 extendingperpendicular to the first direction X and through one of the two endsof the site of the light-emitting element 200, and the virtual line EX2extending perpendicular to the first direction X and through the otherone of the two ends of the site of the light-emitting element 200. Inaddition, the side 100S of the circuit board 100 may refer to a sidethat is adjacent to the light guide plate 110 and extends along thefirst direction X. In some embodiments, the side 100S of the circuitboard 100 may be overlapped with the light guide plate 110 along anormal direction Z of the circuit board 100.

In addition, as shown in FIG. 1 , the first reflective element 310 maysurround the light-emitting element(s) 200, and the second reflectiveelement 320 may be disposed corresponding to the light-reflectingregion(s) 100R. Specifically, in some embodiments, the expression “thefirst reflective element 310 surrounds the light-emitting element(s)200” means that the first reflective element 310 may partially surroundor entirely surround the light-emitting element(s) 200. In someembodiments, the expression “the second reflective element 320 isdisposed corresponding to the light-reflecting region 100R” means thatthe second reflective element 320 is at least partially overlapped withthe light-reflecting region(s) 100R along a normal direction Z of thecircuit board 100.

In some embodiments, the first reflective element 310 may include afirst white material or other reflective materials, and the secondreflective element 320 may include a second white material or otherreflective materials. In some embodiments, the first reflective element310 may include a base layer with white material or other reflectivematerial, but it is not limited thereto. In some embodiments, the baselayer may include polyethylene terephthalate (PET), or another suitablematerial, but it is not limited thereto. In some embodiments, the firstreflective element 310 may be sandwiched between a first adhesive layer(not illustrated) and a second adhesive layer (not illustrated) to forma tape. In other words, the first adhesive layer (not illustrated) andthe second adhesive layer (not illustrated) may be disposed on bothsides of the base layer. In some embodiments, the first reflectiveelement 310 may be attached to the light guide plate 110 through thefirst adhesive layer (not illustrated), and the first reflective element310 (such as the base layer) may be attached to the circuit board 100through the second adhesive layer (not illustrated), but it is notlimited thereto.

In some embodiments, the second white material may include polyimide(PI) with white color or another suitable material, but it is notlimited thereto.

As shown in FIG. 1 and FIG. 2A, in some embodiments, the firstreflective element 310 may be disposed between the light guide plate 110and the second reflective element 320, and the first reflective element310 may partially surround the light-emitting element(s) 200.

As shown in FIG. 1 and FIG. 2B, in some embodiments, the firstreflective element 310 may not be disposed in the light-reflectingregion(s) 100R. In other words, the first reflective element 310 may notbe overlapped with at least one of the plurality of light-reflectingregions 100R along the normal direction Z of the circuit board 100. Insome embodiments, the second reflective element 320 may be disposedcorresponding to the light-reflecting region 100R, and the secondreflective element 320 may not be in contact with the light guide plate110. In this way, the total reflection occurring at an interface of thelight guide plate 110 may not be reduced. Moreover, the secondreflective element 320 disposed corresponding to the light-reflectingregion(s) 100R may enhance the reflection of the light emitted from thelight-emitting element 200.

Moreover, it should be noted that, the first reflective element 310 mayhave a first reflectivity R1, the second reflective element 320 may havea second reflectivity R2, and the first reflectivity R1 and the secondreflectivity R2 satisfy the following formula: 0≤(R1-R2) I/Max (R1,R2)<20%. That is, the ratio of the absolute value of the difference ofthe first reflectivity R1 and the second reflectivity R2 to the maximumvalue of the first reflectivity R1 or the second reflectivity R2 (theone which is greater) may be greater than or equal to 0 and less than20%. In some embodiments, the absolute value of the difference of thefirst reflectivity R1 and the second reflectivity R2 to the maximumvalue of the first reflectivity R1 or the second reflectivity R2 may begreater than or equal to 5% and less than or equal to 18%, for example,6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, or 17%, but it is notlimited thereto.

In some embodiments, the second reflectivity R2 of the second reflectiveelement 320 may be greater than the first reflectivity R1 of the firstreflective element 310. With such a configuration, the second reflectiveelement 320 disposed corresponding to the light-reflecting region 100Rmay have a better reflection performance. The extraction efficiency ofthe backlight module 10 therefore may be improved.

In some embodiments, the first reflectivity R1 of the first reflectiveelement 310 may be greater than or equal to 40% and less than or equalto 65% (i.e. 40%≤first reflectivity R1≤65%), or greater than or equal to45% and less than or equal to 60% (i.e. 45%≤first reflectivity R1≤60%),for example, 50%, or 55%, but it is not limited thereto. In someembodiments, the second reflectivity R2 of the second reflective element320 may be greater than or equal to 45% and less than or equal to 75%(i.e. 45%≤second reflectivity R2≤75%), or greater than or equal to 50%and less than or equal to 70% (i.e. 50%≤second reflectivity R2≤70%), forexample, 55%, 60%, or 65%, but it is not limited thereto.

In an embodiment, the first reflectivity R1 may be 47.13%, the secondreflectivity R2 may be 52.10%, and the value of |(R1−R2)|/Max (R1, R2)is 9.54%. In another embodiment, the first reflectivity R1 may be53.45%, the second reflectivity R2 may be 63.17%, and the value of|(R1−R2)|/Max (R1, R2) may be 15.39%. In another embodiment, the firstreflectivity R1 may be 59%, the second reflectivity R2 may be 62%, andthe value of |(R1−R2)|/Max (R1, R2) may be 4.83%.

It should be noted that when the first reflective element 310 and thesecond reflective element 320 are disposed in the specific configurationas described above and have the reflectivity following the above formula(0≤|(R1−R2)|/Max (R1, R2)<20%), the extraction efficiency of thebacklight module 10 can be improved. Therefore, the performance orreliability of the electronic device can be enhanced.

Moreover, in some embodiments, the first reflectivity R1 may refer to anaverage value obtained by measuring the reflectivity of the firstreflective element 310 three times, and the three reflectivity of thefirst reflective element 310 may be respectively obtained by measuringat different parts of the first reflective element 310. In someembodiments, the second reflectivity R2 may refer to an average valueobtained by measuring the reflectivity of the second reflective element320 in the light-reflecting region 100R three times, and the threereflectivity of the second reflective element 320 may be respectivelyobtained by measuring at different parts of the second reflectiveelement 320 in the light-reflecting region 100R.

In some embodiments, if the tape includes the first adhesive layer (notillustrated), the first reflectivity R1 of the first reflective element310 may be measured after the first adhesive layer is removed. In someembodiments, the first adhesive layer may be removed by scratching, andthe first reflective element 310 may be wiped with ethanol after thescratching, but it is not limited thereto. In some embodiments, a light(such as a testing light) emitted from the halogen lamp is respectivelyreflected by the first reflective element 310 and/or the secondreflective element 320, and the lights reflected by the first reflectiveelement 310 and the second reflective element 320 respectively havecorresponding coordinates (u1′,v1′) and (u2′,v2′) in a CIE 1976 colorspace, and u1′, v1′, u2′ and v2′ satisfy the following formula:0≤√{square root over ((u₂′−u₁′)²+(v₂′−v₁′)²)}≤0.006. The coordinates(u1′,v1′) and (u2′,v2′) represent the chromaticity coordinates of thefirst reflective element 310 and the second reflective element 320,respectively.

It should be noted that, if the value √{square root over((u₂′−u₁′)²+(v₂′−v₁′)²)} of the first reflective element 310 and thesecond reflective element 320 is too large (for example, greater than0.006), the chromaticity difference of the first reflective element 310and the second reflective element 320 may be too large so that the colorof light may be non-uniform.

In some embodiments, the chromaticity coordinate (u1′,v1′) of the firstreflective element 310 may refer to an average value obtained bymeasuring the chromaticity coordinate of the first reflective element310 three times, the three chromaticity coordinates of the firstreflective element 310 may be respectively obtained by measuring atdifferent parts of the first reflective element 310. In someembodiments, the chromaticity coordinate (u2′,v2′) of the secondreflective element 320 may refer to an average value obtained bymeasuring the chromaticity coordinate of the second reflective element320 in the light-reflecting region 100R three times, and the threechromaticity coordinates of the second reflective element 320 may berespectively obtained by measuring at different parts of the secondreflective element 320 in the light-reflecting region 100R.

In some embodiments, the reflectivity and/or the chromaticity of thefirst reflective element 310 (and/or the second reflective element 320)can be measured using an angle analyzer (such as DMS series angleanalyzers, for example DMS 803 or DMS 903) or other instruments. Forexample, the light source of the angle analyzer may include visiblelight (e.g., a wavelength between 380 nm and 780 nm), but it is notlimited thereto. In some embodiments, the light source may includehalogen lamp or other suitable light source, and the spectrum of thelight source is shown in FIG. 3 , but it is not limited thereto. Asdescribed above, in some embodiments the first reflectivity R1 of thefirst reflective element 310 is measured after the first adhesive layer(not illustrated) are removed. The same is applied to the measurement ofchromaticity. In some embodiments, the reflectivity and/or chromaticityof the first reflective element 310 (and/or the second reflectiveelement 320) can be measured using other suitable instruments.

Table 1 and Table 2 show the measurement results of reflectivity andchromaticity of the first reflective element 310 before and after thefirst adhesive layer are removed (with and without first adhesivelayer).

TABLE 1 Sample Sample 1 2 first reflectivity R1 (the first reflectiveelement 53.45% 54.31% 310 with the first adhesive layer) -measurement 1first reflectivity R1 (the first reflective element 54.17% 55.48% 310without the first adhesive layer)-measurement 2 second reflectivity R2(the second reflective element 63.17% 62.08% 320) |(R1 − R2)|/Max (R1,R2) obtained from 15.38% 12.5% measurement 1 |(R1 − R2)|/Max (R1, R2)obtained from 14.24% 10.6% measurement 2

TABLE 2 Sample 1 first reflective first reflective element (with element(without second the first the first reflective adhesive layer) adhesivelayer) element u′ 0.2254 0.2248 0.2267 v′ 0.5243 0.5240 0.5244 u′(second reflective element) − 0.0013 0.0019 u′ (first reflectiveelement) v′ (second reflective element) − 0.0001 0.0004 v′ (firstreflective element) √{square root over ((u′₂ − u′₁)² + (v′₂ − v′₁)²)}0.0013 0.0020

As shown in Table 1 and Table 2, the measurement results of reflectivityand chromaticity of the first reflective element 310 before and afterthe first adhesive layer are removed (with and without the firstadhesive layer) are similar. In addition, the value of |(R1−R2)|/Max(R1, R2) obtained from the first reflectivity R1 before and after thefirst adhesive layer are removed (with and without the first adhesivelayer) and the second reflectivity R2 satisfy the formula describedabove, i.e. 0≤|(R1−R2)|/Max (R1, R2)<20%. The chromaticity coordinatesu1′, v1′, u2′ and v2′ obtained before and after the first adhesive layerare removed (with and without the first adhesive layer)-satisfy thefollowing formula: 0≤√{square root over ((u₂′−u₁′)²+(v₂′−v₁′)²)}≤0.006.

Referring back to FIG. 1 and FIG. 2A, in some embodiments, a part of thesecond reflective element 320 may be overlapped with the firstreflective element 310 from a top view (which is the same as the normaldirection Z of the circuit board 100). With such a configuration, thetolerance for misalignment of the first reflective element 310 and thesecond reflective element 320 may be increased.

In some embodiments, the shape of the second reflective element 320 maybe a T-shape. In some embodiments, part of the second reflective element320 may be disposed in the light-reflecting region 100R, and other partof the second reflective element 320 may be overlapped with thelight-emitting element 200 along the normal direction Z of the circuitboard 10.

In some embodiments, the circuit board 100 may have a side 100S adjacentto the light guide plate 110 and be extending along the first directionX. The side 100S of circuit board 100 may be overlapped with the lightguide plate 110 along the normal direction Z of the circuit board 100.There may be a first distance L_(LB) between one of the plurality oflight-emitting elements 200 and the side 100S of circuit board 100 alongthe second direction Y that is perpendicular to the first direction X.In addition, the second reflective element 320 may have a width L_(PI)along the second direction Y, and the first distance L_(LB) and thewidth L_(PI) of the second reflective element 320 may satisfy thefollowing formula: 0.5≤L_(PI)/L_(LB)≤1. That is, the ratio of the widthL_(PI) to the first distance L_(LB) may be greater than or equal to 0.5and less than or equal to 1, for example, 0.6, 0.7, 0.8, or 0.9, but itis not limited thereto.

It should be noted that if the ratio L_(PI)/L_(LB) is too small (forexample, less than 0.5), the area of the circuit board 100 that isexposed may be too large, and therefore a color-shifting issue mayoccur.

In some embodiments, the first distance L_(LB) may refer to the distancebetween the one of the plurality of light-emitting surfaces 200S and theside 100S of the circuit board 100 along the second direction Y.Moreover, the first distance L_(LB) may refer to an average valueobtained by measuring the distance between the light-emitting surface200S and the side 100S in the light-reflecting region 100R three timesat different positions. In some embodiments, the width L_(PI) may referto an average value obtained by measuring the width of the secondreflective element 320 along the second direction Y three times, thethree widths L_(PI) may be respectively obtained by measuring atdifferent positions. In addition, when the second reflective element 320has the other part overlapped with the light-emitting element 200, thewidths of the second reflective element 320 that are selected forcalculating the average value should exclude the other part of thesecond reflective element 320 overlapped with the light-emitting element200 along the normal direction Z of the circuit board 10.

In some embodiments, the second reflective element 320 may have a firstside 320S-1, and the first side 320S-1 may be adjacent to thelight-emitting element(s) 200 and extends along the first direction X.In some embodiments, there may be a distance d between the first side320S-1 of the second reflective element 320 and one of the plurality oflight-emitting elements 200 along the second direction Y that isperpendicular to the first direction X, and the distance d satisfies thefollowing formula: 0≤d≤0.5 mm. That is, the distance d between the firstside 320S-1 and the light-emitting element 200 may be greater than orequal to 0 and less than or equal to 0.5 mm, for example, 0.1 mm, 0.2mm, 0.3 mm, or 0.4 mm, but it is not limited thereto. It should be notedthat, if the distance d between the first side 320S-1 and thelight-emitting element 200 is too large (for example, greater than 0.5mm), the area of the circuit board 100 that is exposed may be too largeand therefore a color band may occur near the light-emitting surface200S or the backlight module 10 may have a color-shifting issue.

Moreover, the distance d may refer to an average value obtained bymeasuring the distance between the first side 320S-1 and thelight-emitting surface 200S in the light-reflecting region 100R threetimes at different positions, the three distances d may be respectivelyobtained by measuring at different positions. In addition, the distancesd that are selected for calculating the average value should exclude theother part of the second reflective element 320 overlapped with thelight-emitting element 200 along the normal direction Z of the circuitboard 10.

In some embodiments, the second reflective element 320 may have a secondside 320S-2. The second side 320S-2 is opposite to the first side 320S-1and extends along the first direction X. In some embodiments, the secondside 320S-2 of the second reflective element 320 may be adjacent to theside 100S of the circuit board 100. In some embodiments, there may be adistance d2 between the second side 320S-2 and the side 100S of thecircuit board 100 in the second direction Y perpendicular to the firstdirection X. In some embodiments, the distance d2 may be greater than 0,that is, the second side 320S-2 may not be overlapped with the side 100Salong the normal direction Z of the circuit board 100. For example, theside 100S may be protrude more than the second side 320S-2 in the seconddirection Y.

As described above, in some embodiments, the second reflective element320 may not be in contact with the light guide plate 110 in thelight-reflecting region 100R (as shown in FIG. 2B). Specifically, theremay be a distance d1 between the second reflective element 320 and thelight guide plate 110 along the normal direction Z of the circuit board100. The distance d1 may be substantially the same as the thickness ofthe first reflective element 310 (as shown in FIG. 2A) along the normaldirection Z of the circuit board 100. In some embodiments, the distanced1 may satisfy the following formula: 0<distance d1≤200 μm. In someembodiments, the distance d1 may be greater than or equal to 50 μm andless than or equal to 100 μm, for example, 60 μm, 70 μm, 80 μm, or 90μm, but it is not limited thereto.

It should be noted that if the distance d1 between the second reflectiveelement 320 and the light guide plate 110 is too large (for example,greater than 200 μm), the amount of the light transmitting from thelight-emitting element 200 to the light guide plate 110 may bedecreased.

In some embodiments, the distance d1 may refer to an average valueobtained by measuring the distance between the second reflective element320 and the light guide plate 110 in any cross-section three times, thethree distances d1 may be respectively obtained by measuring atdifferent positions.

Referring to FIG. 2A and FIG. 2B, in some embodiments, the material ofthe third reflective element 120 may include metal, white materials(such white ink, white tape), other suitable reflective materials or acombination thereof, but it is not limited thereto. The third reflectiveelement 120 may have a third reflectivity R3. In some embodiments, thethird reflectivity R3 of the third reflective element 120 may be greaterthan the second reflectivity R2 of the second reflective element 320. Itshould be note that if the third reflectivity R3 is less than the secondreflectivity R2, a bright band may be formed, and the optical quality ofthe backlight module 10 may be affected.

In some embodiments, the second reflectivity R2 and the thirdreflectivity R3 satisfy the following formula: 0≤|(R2-R3)|/Max(R2,R3)<50%. That is, the ratio of the absolute value of the differenceof the second reflectivity R2 and the third reflectivity R3 to themaximum value of the second reflectivity R2 or the third reflectivity R3(the one which is greater) may be greater than or equal to 0 and lessthan 50%. In some embodiments, the value of |(R2-R3)|/Max (R2,R3) may begreater than or equal to 10% and less than or equal to 45%, for example,15%, 20%, 25%, 30%, 35%, or 40%, but it is not limited thereto.

More specifically, in an embodiment, the second reflectivity R2 is52.10%, the third reflectivity R3 is 99%, and the value of |(R2-R3)|/Max(R2,R3) is 47.37%. In another embodiment, the second reflectivity R2 maybe 63.17%, the third reflectivity R3 is 99%, and the value of|(R2-R3)|/Max (R2,R3) may be 36.19%.

It should be noted that if the value of |(R2-R3)|/Max (R2,R3) is toolarge (for example, greater than 50%), the extraction efficiency of thebacklight module 10 may be decreased. Moreover, a bright band or darkband may be formed, and the optical quality of the backlight module 10may be affected.

Moreover, in some embodiments, the third reflectivity R3 may refer to anaverage value obtained by measuring the reflectivity of the thirdreflective element 120 three times, the three times reflectivity of thethird reflective element 120 may be respectively obtained by measuringat different parts of the third reflective element 120.

Next, refer to FIG. 4 , which is a cross-sectional diagram of thelight-emitting element 200 of the backlight module 10 in accordance withsome embodiments of the present disclosure. FIG. 4 illustrates anexemplary structure of the light-emitting element 200, but the presentdisclosure is not limited thereto.

As described above, the light-emitting element 200 may be an inorganiclight-emitting diode in some embodiments. The light-emitting element 200may include a substrate 202, a first semiconductor layer 206, a quantumwell layer 208, a second semiconductor layer 210, a conductive layer212, a first electrode 214 a and a second electrode 214 b, but it is notlimited thereto.

Furthermore, the first electrode 214 a and the second electrode 214 b ofthe light-emitting element 200 may be electrically connected to thecircuit board 100 through the conductive pads 216 of the circuit board100 (shown in FIG. 1 and FIG. 2B).

In some embodiments, the first semiconductor layer 206 may be one ofn-type semiconductor or p-type semiconductor, and the secondsemiconductor layer 210 may be another of n-type semiconductor or p-typesemiconductor. The n-type semiconductor may include, but is not limitedto, gallium nitride (n-GaN) or aluminum indium phosphide (n-AlInP) thatis doped with tetravalent atoms. The p-type semiconductor may include,but is not limited to, gallium nitride (p-GaN) or aluminum indiumphosphide (p-AlInP) that is doped with divalent atoms. In someembodiments, the quantum well layer 208 may include a single quantumwell (SQW) or a multiple quantum well (MQW). The material of the quantumlayer 208 may include, but is not limited to, gallium nitride, aluminumindium phosphide (AlInP), indium gallium nitride (InGaN), or acombination thereof.

In some embodiments, the material of the conductive layer 212 mayinclude transparent conductive martial, such as indium tin oxide (ITO),tin oxide (SnO), zinc oxide (ZnO), indium zinc oxide (IZO), indiumgallium zinc oxide (IGZO), indium tin zinc oxide (ITZO), antimony tinoxide (ATO), antimony zinc oxide (AZO), another suitable transparentconductive material, or a combination thereof. In some embodiments, thematerial of the first electrode 214 a and the second electrode 214 b mayinclude a metallic conductive material.

It should be understood that the light-emitting element 200 may includethe packaging component (not illustrated) in FIG. 4 . FIG. 4 illustratesa bare die of light-emitting element 200. In addition, although thelight-emitting element 200 illustrated in FIG. 4 has a vertical typestructure, the light-emitting element 200 may have a flip-chip typestructure in some other embodiments.

In the following description, various aspects of the backlight moduleare shown. It should be understood that the same or similar componentsor elements in above and below contexts are represented by the same orsimilar reference numerals. The materials, manufacturing methods andfunctions of these components or elements are the same or similar tothose described above, and thus will not be repeated herein.

Refer to FIG. 5 , which is a partial top-view diagram of a backlightmodule 20 in accordance with some other embodiments of the presentdisclosure. As shown in FIG. 5 , in some embodiments, there may be nodistance between the first side 320S-1 of the second reflective element320 and one of the light-emitting surfaces 200S in one of thelight-reflecting regions 100R in the second direction Y. In other words,part of the second reflective element 320 (such as the second side320S-2) may be substantially overlapped with one of the plurality of thelight-emitting elements 200 along the normal direction Z of the circuitboard 100. Therefore, the distance d2 between the second side 320S-2 andthe side 100S of circuit board 100 in the second direction Y may bezero. Furthermore, the first distance L_(LB) between one of thelight-emitting elements 200 and the side 100S may be substantially thesame as the width L_(PI) of the second reflective element 320 in thesecond direction Y.

Refer to FIG. 6 , which is a partial top-view diagram of a backlightmodule 30 in accordance with some other embodiments of the presentdisclosure. As shown in FIG. 6 , in some embodiments, the shape of thesecond reflective element 320 may be substantially a strip shape. Thedistance d may exist between the first side 320S-1 of the secondreflective element 320 and the light-emitting surface 200S in the seconddirection Y. The first distance L_(LB) may be greater than the widthL_(PI), that is, the distance d2 may exist between the second side320S-2 and the side 100S in the second direction Y. In some embodiments,the first reflective element 310 may not be overlapped with thelight-reflecting region 100R along the normal direction Z of the circuitboard 100.

Refer to FIG. 7 , which is a partial top-view diagram of a backlightmodule 40 in accordance with some other embodiments of the presentdisclosure. As shown in FIG. 7 , in some embodiments, there may be nodistance between the first side 320S-1 of the second reflective element320 and the light-emitting surface 200S of the light-emitting element200 in the light-reflecting region 100R in the second direction Y. Insome embodiments, the second reflective element 320 may be partiallyoverlapped with the light-emitting element 200 along the normaldirection Z of the circuit board 100. In addition, the first reflectiveelement 310 may slightly protrude outward the side 100S of circuit board100 (or the second side 320S-2 of the second reflective element 320) inthe second direction Y. In other words, part of the first reflectiveelement 310 may not be overlapped with the circuit board 100 along thenormal direction Z of the circuit board 100. In some embodiments, thefirst distance L_(LB) between the light-emitting element 200 and theside 100S may be substantially the same as the width L_(PI) of thesecond reflective element 320 in the second direction Y. In someembodiments, the first reflective element 310 may not be overlapped withthe light-reflecting region 100R along the normal direction Z of thecircuit board 100.

Refer to FIG. 8 , which is a partial top-view diagram of a backlightmodule 50 in accordance with some other embodiments of the presentdisclosure. As shown in FIG. 8 , in some embodiments, the firstreflective element 310 may surround the light-emitting element(s) 200and overlapped with at least part of the light-reflecting region(s) 100Ralong the normal direction Z of the circuit board 100. The firstreflective element 310 may be overlapped with the side 100S of thecircuit board 100 along the normal direction Z of the circuit board 100.In some embodiments, the shape of the first reflective element 310 maybe a grid shape, the first reflective element 310 may have the aperturesAP, and the second reflective element 320 may be overlapped with theapertures AP of the first reflective element 310 along the normaldirection Z of the circuit board 100.

Refer to FIG. 9 , which is a partial top-view diagram of a backlightmodule 60 in accordance with some other embodiments of the presentdisclosure. As shown in FIG. 9 , in some embodiments, the firstreflective element 310 may have a plurality of sub-parts 310S separatedfrom each other, a shape of at least one of the plurality of sub-parts310S may be a tapered shape or a bullet shape. The sub-parts 310S may bedisposed between the light-emitting elements 200 respectively and arenot overlapped with the light-reflecting region 100R along the normaldirection Z of the circuit board 100. In some embodiments, the sub-parts310S of the first reflective element 310 and the light-emitting elements200 may be alternately arranged along the first direction X. Inaddition, the circuit board 100 may have a first part P1 and a secondpart P2 connected with the first part P1, the first part P1 may extendalong the first direction X, and a second part P2 may extend along thesecond direction Y. The light-emitting elements 200 may be disposed onthe first part P1, and the second part P2 may be away from the side 100Sof the circuit board 100. In some embodiments, a part of the secondreflective element 320 may be overlapped with the first part P1 alongthe normal direction Z of the circuit board 100, but the part of thesecond reflective element 320 may not be overlapped with the conductivepads 216 of the circuit board 100 (shown in FIG. 1 ) along the normaldirection Z of the circuit board 100. In some embodiments, the secondreflective element 320 may be overlapped with a part of the second partP2 along the normal direction Z of the circuit board 100, and anotherpart of the second part P2, which is not overlapped with the secondreflective element 320 may be used as a connector 400. In someembodiments, the external signals can transmit to the light-emittingelements 200 through the connector 400.

Refer to FIG. 10 , which is a partial top-view diagram of a backlightmodule 70 in accordance with some other embodiments of the presentdisclosure. As shown in FIG. 10 , in some embodiments, the shape of thesecond reflective element 320 may be a strip shape. There may be nodistance between the first side 320S-1 of the second reflective element320 and the light-emitting surface 200S of one of the light-emittingelements 20 in the light-reflecting region 100R. In some embodiments,part of the first reflective element 310 may protrude outward the secondside 320S-2 of the second reflective element 320 in the second directionY. In other words, a minimum distance d3 between the first reflectiveelement 310 and side 100S in the second direction Y is less than thedistance d2 between the second side 320S-2 and the side 100S of thecircuit board 100 in the second direction Y. Furthermore, the firstdistance L_(LB) between the light-emitting element 200 and the side 100Smay be greater than the width L_(PI) of the second reflective element320. In some embodiments, the first reflective element 310 may not beoverlapped with the side 100S of the circuit board 100 along the normaldirection Z of the circuit board 100, the first reflective element 310may not protrude outward than the side 100S of the circuit board 100 inthe second direction Y. In other words, there is the minimum distance d3between the first reflective element 310 and side 100S.

To summarize the above, in accordance with some embodiments of thepresent disclosure, the backlight module includes the first reflectiveelement and the second reflective element that are arranged in aspecific configuration. In addition, the reflectivity of the firstreflective element and the second reflective element are designed tofollow a certain formula. With such a specific configuration, theextraction efficiency of the backlight module can be improved.Therefore, the performance or reliability of the electronic device canbe enhanced.

Although some embodiments of the present disclosure and their advantageshave been described in detail, it should be understood that variouschanges, substitutions and alterations can be made herein withoutdeparting from the spirit and scope of the disclosure as defined by theappended claims. The features of the various embodiments can be used inany combination as long as they do not depart from the spirit and scopeof the present disclosure. Moreover, the scope of the presentapplication is not intended to be limited to the particular embodimentsof the process, machine, manufacture, composition of matter, means,methods and steps described in the specification. As one of ordinaryskill in the art will readily appreciate from the present disclosure,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed, thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present disclosure. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods or steps.In addition, each claim constitutes an individual embodiment, and theclaimed scope of the present disclosure includes the combinations of theclaims and embodiments. The scope of protection of present disclosure issubject to the definition of the scope of the appended claims. Anyembodiment or claim of the present disclosure does not need to meet allthe purposes, advantages, and features disclosed in the presentdisclosure.

What is claimed is:
 1. A light-emitting device, comprising: a circuitboard; a plurality of light-emitting elements arranged on the circuitboard; a first reflective element disposed on the circuit board; and asecond reflective element disposed on the circuit board; wherein thefirst reflective element has a first reflectivity R1, the secondreflective element has a second reflectivity R2, the first reflectivityR1 is different from the second reflectivity R2, and the firstreflectivity R1 and the second reflectivity R2 satisfy the followingformula:0<|(R1−R2)|/Max(R1,R2)<20%.
 2. The light-emitting device as claimed inclaim 1, wherein the second reflectivity R2 is greater than the firstreflectivity R1.
 3. The light-emitting device as claimed in claim 1,wherein the first reflective element comprises a first white material,and the second reflective element comprises a second white material. 4.The light-emitting device as claimed in claim 1, wherein a part of thesecond reflective element is overlapped with the first reflectiveelement along a normal direction of the circuit board.
 5. Thelight-emitting device as claimed in claim 1, wherein a shape of thesecond reflective element is a strip shape, T-shape or a grid shape. 6.The light-emitting device as claimed in claim 1, wherein a part of thesecond reflective element is overlapped with one of the plurality oflight-emitting elements along a normal direction of the circuit board.7. The light-emitting device as claimed in claim 1, wherein the firstreflective element having a plurality of sub-parts separated from eachother, and the plurality of sub-parts and the plurality oflight-emitting elements are alternately arranged on the circuit board.8. The light-emitting device as claimed in claim 7, wherein at least oneof the plurality of sub-parts has a tapered shape or a bullet shape. 9.The light-emitting device as claimed in claim 1, further comprising athird reflective element disposed adjacent to the circuit board, whereinthe third reflective element has a third reflectivity R3 greater thanthe second reflectivity R2.
 10. The light-emitting device as claimed inclaim 9, wherein the second reflectivity R2 and the third reflectivityR3 satisfy the following formula:0≤√{square root over ((u ₂ ′−u ₁′)²+(v ₂ ′−v ₁′)²)}≤0.006.
 11. Thelight-emitting device as claimed in claim 1, wherein the firstreflective element surrounds at least one of the plurality oflight-emitting elements.
 12. The light-emitting device as claimed inclaim 1, wherein a light emitted from a halogen lamp is respectivelyreflected by the first reflective element and the second reflectiveelement, and lights reflected by the first reflective element and thesecond reflective element respectively have corresponding coordinates(u1′,v1′) and (u2′,v2′) in a CIE1976 color space, and u1′, v1′, u2′ andv2′ satisfy the following formula:0≤√{square root over ((u ₂ ′−u ₁′)²+(v ₂ ′−v ₁′)²)}≤0.006.
 13. Thelight-emitting device as claimed in claim 1, further comprising a lightguide plate disposed on the circuit board, wherein the circuit board hasa side overlapping with the light guide plate, a first distance L_(LB)is between one of the plurality of light-emitting elements and the sideof the circuit board, the second reflective element has a width L_(PI)and the first distance L_(LB) and the width L_(PI) of the secondreflective element satisfy the following formula:0.5≤L _(PI) /L _(LB)≤1.
 14. The light-emitting device as claimed inclaim 1, wherein the second reflective element has a first side adjacentto the plurality of light-emitting elements and extending along a firstdirection, a distance d is between the first side of the secondreflective element and one of the plurality of light-emitting elementsalong a second direction perpendicular to the first direction, and thedistance d satisfies the following formula:0≤d≤0.5 mm.
 15. The light-emitting device as claimed in claim 14,wherein the second reflective element has a second side opposite to thefirst side and extending along the first direction, and a distance d2 isbetween the second side and a side of the circuit board along the seconddirection.
 16. The light-emitting device as claimed in claim 1, furthercomprising a light guide plate disposed on the circuit board, wherein adistance d1 between the second reflective element and the light guideplate along a normal direction of the circuit board satisfies thefollowing formula:0<d1≤200μm.
 17. An electronic device, comprising: a display panel; and alight-emitting device disposed below the display panel and comprising: acircuit board; a plurality of light-emitting elements arranged on thecircuit board; a first reflective element disposed on the circuit board;and a second reflective element disposed on the circuit board; whereinthe first reflective element has a first reflectivity R1, the secondreflective element has a second reflectivity R2, the first reflectivityR1 is different from the second reflectivity R2, and the firstreflectivity R1 and the second reflectivity R2 satisfy the followingformula:0<|(R1−R2)|/Max(R1,R2)<20%.
 18. The electronic device as claimed inclaim 17, wherein the second reflectivity R2 is greater than the firstreflectivity R1.
 19. The electronic device as claimed in claim 17,wherein the light-emitting device further comprises a third reflectiveelement disposed adjacent to the circuit board, and the third reflectiveelement has a third reflectivity R3 greater than the second reflectivityR2.
 20. The electronic device as claimed in claim 17, wherein a lightemitted from a halogen lamp is respectively reflected by the firstreflective element and the second reflective element, and lightsreflected by the first reflective element and the second reflectiveelement respectively have corresponding coordinates (u1′,v1′) and(u2′,v2′) in a CIE1976 color space, and u1′, v1′, u2′ and v2′ satisfythe following formula:0≤√{square root over ((u ₂ ′−u ₁′)²+(v ₂ ′−v ₁′)²)}≤0.006.